Silver-cadmium oxide alloys having periodic precipitation

ABSTRACT

IN ACCORDANCE WITH THE PRESENT INVENTION, IT HAS BEEN FOUND THAT IF AT LEAST ONE ADDITIVE SELECTED FROM THE GROUP CONSISTING OF BERYLLIUM, MAGNESIUM, ALUMINUM, SCANDIUM, YTTRIUM, LANTHANUM AND NEODYMIUM IS ADDED TO A SILVER CADMIUM ALLOY IN AN AMOUNT FROM ABOUT 0.005 TO ABOUT 1 WEIGHT PERCENT, PRECIPITATION OF THE CADMIUM OXIDE OC-   CURS IN LAYERS WHICH ARE PARALLEL TO THE OXIDATION FRONT. THIS RESULTS IN DIRECTIONAL PROPERTIES BEING ACHIEVED IN THE MATERIAL, INCLUDING ELECTRICAL CONDUCITVITY AND MECHANICAL PROPERTIES.

SQPLQ 1972 R. H. KROCK- ETAL 3,694,198

SILVER-CADMIUM OXIDE ALLOYS HAVING PERIODIC PRECIPITATION Filed Feb. 19, 1971 2 Sheets-Sheet 1 INVENTORS RICHARD H. KROCK YUAN SHOU SHEN EDWARD. J. ZDANUK ATTORNEY 'P 1972 R. H. KROCK ETA!- SILVER-CADMIUM OXIDE ALLOYS HAVING PERIODIC PRECIPITATION Filed Feb. 19, 1971 2 Sheets-Sheet 2 K NU Y O N E R HA N O SD R T Z 0 N T E OJ T W D A s I M RU H M A l UD RY United States Patent 3,694,198 SILVER-CADMIUM OXIDE ALLOYS HAVING PERIODIC PRECIPITATION Richard H. Krock, Weston, Yuan Shou Shen, Reading,

and Edward J. Zdanuk, Burlington, Mass., assignors to P. R. Mallory & Co. Inc., Indianapolis, Ind.

Filed Feb. 19, 1971, Ser. No. 116,820 Int. Cl. C22c /00 US. Cl. 75-173 R 27 Claims ABSTRACT OF THE DISCLOSURE material, including electrical conductivity and mechanical properties.

BACKGROUND Up to the present time most of the commercial electrical contact material of the silver-cadmium oxide type is produced according to the teachings of US. Patent 2,539,298, issued July 28, 1945 to A. S. Doty. In this patent, the cadmium oxide is precipitated in the silver matrix in a random fashion.

However, the arc erosion rate of this material is not as low as desired in service. Furthermore, it is desired that the electrical conductivity of the material be as high as possible, while at the same time reducing the arc erosion rate.

In application Ser. No. 88,620, filed Nov. 12, 1970, assigned to the same assignee as the present application, We described electrical contact materials contacting additives in silver cadmium oxide material. While the compositions in that patent application result in a reduced arc erosion THE DRAWINGS FIG. 1 is a view of a silver-cadmium oxide microstructure containing 9 Weight percent cadmium at 545x, illustrating a random distribution of cadmium oxide in the silver matrix.

FIG. 2 is a view of the microstructure in a 9 weight percent cadmium silver matrix material containing 0.05 weight percent aluminum at 545 X.

FIG. 3 is a view of a silver 9 Weight percent cadmium material containing 0.1 percent by weight beryllium at 545 X FIG. 4 is a view of a silver 9 weight percent cadmium material with 0.10 weight percent yttrium microstructure at 545 X FIG. 5 is a view of a silver 9 weight percent cadmium with 0.3% magnesium material at 260x.

FIG. 6 is a view of a silver 9 weight percent cadmium 0.2 percent scandium material at 390x.

ice

SUMMARY OF THE INVENTION In accordance with the present invention, it has been found that if at least one additive selected from the group consisting of beryllium, magnesium, aluminum, scandium, yttrium, lanthanum and neodymium is added to a silver cadmium alloy in an amount from about 0.005 to about 1 weight percent, precipitation of the cadmium oxide occurs in layers which are parallel to the oxidation front. This results in directional properties being achieved in the material, including electrical conductivity and mechanical properties.

DETAILED DESCRIPTION The cadmium content in the alloys of the present invention can vary from as little as about 1 percent up to about 30 percent by weight cadmium. The greater amount of cadmium present, the lower is the electrical conductivity of the material. Preferably, the cadmium content is from 1 percent up to about 15 percent by weight.

In accordance with the present invention at least one element selected from the group consisting of beryllium, magnesium, aluminum, scandium, yttrium, lanthanum and neodymium in an amount from about 0.01 percent by weight up to about 1 percent by weight.

Attention is directed to FIG. 3 of the drawings. Here a silver 9 percent cadmium alloy containing 0.01 percent beryllium is shown. The oxidation was carried out from the surface at the top of the photomicrograph and thus it can be seen that the bands of cadmium oxide precipitate increase in thickness as one looks down towards the bottom of the photomicrograph. In fact, at the very lower portion of the photomicrograph the periodic precipitation at least partially ceases and some random precipitation may be observed. It is within the scope of the present structures where the precipitation is not substantially all parallel to the oxidation front, but at least a portion of the precipitate is in bands which are parallel to the oxidation front.

The range for beryllium additions broadly is from about 0.01 up to about 0.5 Weight percent. Preferably, the range is from 0.05 weight percent up to about 0.2 weight percent.

-In most, if not all applications, the periodic precipitation band should be perpendicular to the contact surface. Thus, the current can travel parallel to the precipitation through the easy conducting paths which are found in the silver matrix between the layers of precipitates.

In the event that the additive is aluminum, the amount to be added may be within the range of 0.005 percent up to about 0.5 percent by weight. Preferably, the amount of aluminum is within the range of 0.001 to 0.2 percent. FIG. 2 gives a particularly effective illustration of the periodic precipitation of the present invention. Here three grains A, B and C come together and the three grain boundaries are viewable in the photomicrograph. It will be apparent that notwithstanding the grain intersection, the periodic precipitation has continued across the grain and still the relationship of precipitation parallel to the oxidation front is maintained.

For magnesium the broad range of 0.05 up to about 1 percent and the preferred range is from 0.1 to 0.5 percent.

FIG. 5 shows a photomicrograph with 0.3 percent magalong with the periodic precipitation. In the case of additives such as yttrium wherein there is some non-parallel precipitation, the extent of property improvement in a direction parallel to the band is reduced to the extent of the non-periodic precipitation. For example, conductivity is not as high in a direction parallel to the bands in the case of yttrium as is observed with respect to additives wherein there is substantially no non-periodic precipitation.

In the case of scandium, the broad range is from 0.05 to 0.5 percent, preferably, 0.1 to 0.2 percent. As can be seen from FIG. 6 the precipitation is substantially all periodic and there is a relatively little precipitation which is not parallel to the oxygen front.

The elements lanthanum and neodymium also may be added. The range to be utilized is from about 0.1 to about 0.7 percent by weight for either of these elements. It should be pointed out, however, that additions of these elements only result in precipitation in which the precipitate is partially periodic. There is a considerable amount of precipitation which does not occur in a direction parallel to the oxygen front.

The reason why some elements result in periodic precipitation and others do not, and the reason some elements give only partial periodic precipitation is not known. It is assumed that it lies somewhere in the mechanism of nucleation and growth, but to date the particular mechanisms have not been determined.

More than one additive may be employed if the total of such additives is small, generally not above about 0.2% by weight depending upon the additives. However, in general more than one additive is not required and is not preferred because of cost.

In accordance with the present invention, if a silvercadmium oxide material having an additive as above indicated is utilized with the periodic precipitation perpendicular to the contact face, an improvement in electrical conductivity in a direction perpendicular to the contact face, (i.e., parallel to the periodic precipitation) is obtained) is obtained. In general, for cadmium oxide contents of about 10 weight percent and higher the electrical conductivity will generally be increased at least percent IACS (International Annealed Copper Standard). Often, improvement up to percent and higher can be achieved particularly in the higher Weight percent cadmium oxide contents. The amount of improvement increases With higher amounts of cadmium oxide and decreases with lower amounts of cadmium oxide. In fact, at cadmium oxide contents below about 5 weight percent, the amount of improvement is not generally significant for most applications. At 5 weight percent cadmium oxide improvements of about 3 percent IACS are often observed.

The following table summarizes the improvements which may be expected in accordance with the present invention.

Minimum conduc- General Random, tivity improvement percent; (percent conductivity IACS IACS) (IACS) In the event that one desires to prepare the contact face from a single element, diameters up to about one quarter of an inch can be produced by masking off both faces and then diffusing in oxygen along the longitudinal axis of the wire. However, it may be more practical for many applications to build up the contact face from a plurality of long thin sheets or wires which have been previously treated in an appropriate oxidizing atmosphere to achieve the periodic precipitation. For example, a plurality of sheets or wires which have been treated to obtain periodic 4 precipitation may be mechanically worked such as by swaging or rolling to achieve an effective contact face.

An alternative procedure is to treat a thin sheet for periodic precipitation in an oxidizing atmosphere of air or air enriched with oxygen and then roll the sheet into a coil. Then, the sheet may be mechanically worked to achieve a desired contact face material.

Other methods of achieving a satisfactory contact face with the periodic precipitation of the present invention will be apparent to those skilled in the fabricating and mechanical working arts.

With respect to preparing the alloys of the present in vention, a silver melt is prepared and then the particular additive and cadmium are added to this melt. Preferably, the cadmium is added last to the melt because of the high vapor pressure. However, if desired, and if available an alloy of silver and one of the additives of the present invention could be melted and then the cadmium added to such an alloy. A final alternative involves melting a silver-cadmium alloy and then adding the additive. This latter alternative is less preferred because of losses of cadmium and often segregation results during casting. The melt may be formed in air in a vacuum or in an inert atmosphere. A vacuum is not preferred because cadmium would be eliminated at a greater rate under vacuum conditions. Air is also less preferred because of some oxidation taking place during the melting operation. Therefore, the preferred method is to carry out the melting in an inert atmosphere such as argon, krypton, helium and/or mixtures thereof.

The melt may be sand cast, but preferably a permanent mold is utilized during casting. Again, it is preferably carried out in an inert atmosphere. There is no restriction as to size or shape of the casting, other than ordinary practical considerations of handling and mill capability. During casting, it is preferred to stir the metal being cast, for example with induction stirring apparatus or mechanical stirring.

After the castings are formed the ingots are mechanically treated to obtain either sheet or rod according to conventional treatments for silver-cadmium alloys. This may involve hot and/ or cold rolling and/ or extrusion with or without intermediate anneals. These practices are conventional in the art. In the case of wire, for example, extrusion and drawing is often utilized.

Preferably, the mechanical reduction is continued to a point that in the case of rod a diameter of less than about /2 inch is present and in the case of sheet a cross sectional thickness of less than about /2 inch is achieved. If other cross sectional shapes are obtained the smallest cross sectional shape should not be in excess of about /2 inch. Preferably, the thickness is considerably less than /2 inch, and in general the thinner geometry is more favorable from the standpoint of the oxygen diffusion process for forming the periodic precipitation parallel layers.

If the length to cross sectional area dimension is quite large, above about 20, it is not necessary to mask off the ends at which diffusion is not to take place, rather these may be barely cut off after the diffusion operation. However, if the articles to be treated do not have a high length to cross sectional dimension, the ends where diffusion does not take place are preferably masked. This masking may be done for instance by applying nickel to the ends to be masked, for example by plating or vapor deposition or plasma spray. Another masking method involves applying an organic coating to the ends to be masked. Other masking techniques will be apparent to those skilled in the art.

The next step in the process is to diffuse oxygen into the alloys of the present invention to form the periodic precipitation. In general, the temperature range for the diffusion process is from about 650 to about 900 C. Temperatures below about 650 C. require such long diffusion times of up to a year or longer, that they are impractical for commercial operation. Temperatures above about 900 C. run the risk of melting the alloy. Generally, the times'for oxidation are from as low as about 4 hours up to a week or longer. Longer times are required at lower temperature and when greater thicknesses are desired. The treatment time can be reduced by the use of an atmosphere containing a greater amount of oxidation than does air. The use of such an oxygen enriched atmosphere will depend upon the availability of such an atmosphere and the economics of the particular process. For example, to obtain a periodic precipitation extending about A; inch from the diffusing surface, a time of 3 days was required at 800 C. in an oxygen enriched atmosphere to diffuse in aluminum, and beryllium.

After treatment in an oxidizing atmosphere the present alloy is treated in a conventional manner to obtain desired shape, for example by mechanical working, or other shaping operations, it necessary.

As mentioned above, if desired in order to build up a large contact face, it may be necessary to utilize a plurality of rods or sheet members and mechanically work them to achieve a large face for example by rolling or swaging. Particular contact elements may then be cut to a particular size desired.

In addition to achieving the improved conductivity in a direction parallel to the periodic precipitation, in all cases in which a reduced arc erosion rate was also achieved with all the aditives of the present invention except aluminum. In general, this improvement is at least 15% based on the arc erosion weight loss over a period of time and in general an improvement of at least 15% is achieved in cyclic contentlrnake and break tests.

We claim: j :5

1. A silver based alloy containing from about 1 to about 25 weight percent cadmium and at least one additive selected from beryllium, magnesium, aluminum, scandium, yttrium, lanthanum and neodymium in an amount from about 0.005 to about 1 percent by weight, said alloy having a microstructure in which cadmium oxide is at least partly precipitated in substantially parallel layers in a silver matrix.

2. -An alloy according to claim 1 wherein the additive is selected from beryllium, magnesium, aluminum, scandium and yttrium and wherein said cadmium oxide precipitate is substantially entirely in parallel layers for at least a portion of the alloy.

3. An alloy according to claim 1 wherein said additive is beryllium in an amount from 0.01 to about 0.5 percent.

4. An alloy according to claim 1 wherein said additive is aluminum in an amount from 0.005 to about 0.5 percent.

5. An alloy according to claim 1 wherein said additive is magnesium in an amount from 0.05 to about 1 percent.

6. An alloy according to claim 1 wherein the alloy is yttrium in an amount from about 0.1 to 1 percent.

7. An alloy according to claim 1 wherein the alloy is scandium in an amount from about 0.05 to 0.5 percent.

8. An alloy according to claim 1 wherein the alloy is lanthanum in an amount from about 0.01 to 0.7 percent.

9. An alloy according to claim 1 wherein the alloy is neodymium in an amount from about 0.1 to 0.7 percent.

10. A method of forming a silver-cadmium alloy having periodic precipitation comprising: forming an alloy of silver containing from about 1 to about 25 percent cadmium and at least one additive selected from the group consisting of beryllium, aluminum, magnesium, scandium, yttrium, lanthanum and neodymium in an amount from 0.005 to about 1 percent by weight and oxidizing said alloy at a temperature from about 650 to 6 about 900 C. for a time sufiicient to obtain at least some precipitation of cadmium oxide in a direction substantially perpendicular to the surface of said alloy.

11. A method according to claim 10 wherein an oxygen enriched atmosphere is utilized.

12. A method according to claim 10 wherein the alloy oxidized does not have a cross sectional thickness in excess of about /2 inch.

13. A method according to claim 10 wherein an ingot is formed of said alloy and wherein said alloy is mechanically treated to achieve a cross sectional thickness of below about /2 inch.

14. A method according to claim 10 wherein said alloy is formed by casting.

15. A method according to claim 14 wherein said casting operating is carried out while stirring.

16. A method according to claim 10 wherein a melt is formed of silver and cadmium and said additive is added thereto prior to casting.

17. A method according to claim 10 wherein the oxidizing time is at least 4 hours.

18. A method according to claim 10 wherein a plurality of elements are assembled to obtain a contact face.

19. An electrical contact comprising a silNer base alloy containing from about 1 to about 25 weight percent cadmium and at least one additive selected from beryllium, magnesium, aluminum, scandium, yttrium, lanthanum and neodymium in an amount from about 0.005 to about 1 percent by weight, said alloy having a microstructure in which cadmium oxide is at least partly precipitated in substantially parallel layers in a silver matrix.

20. An electrical contact according to claim 19 wherein the additive is selected from beryllium, magnesium, aluminum, scandium and yttrium and wherein said cadmium oxide precipitate is substantially entirely in parallel layers for at least a portion of the alloy.

21. An electrical contact according to claim 19 wherein said additive is beryllium in an amount from 0.01 to about 0.5 percent.

22. An electrical contact according to claim 19 wherein said additive is aluminum in an amount from 0.005 to about 0.5 percent.

23. An electrical contact according to claim 19 wherein said additive is magnesium in an amount from 0.05 to about 1 percent.

24. An electrical contact according to claim 19 where in the additive is yttrium in an amount from about 0.1 to 1 percent.

25. An electrical contact according to claim 19 wherein the additive is scandium in an amount from about 0.05 to 0.5 percent.

26. An electrical contact according to claim 19 wherein the additive is lanthanum in an amount from about 0.01 to 0.7 percent.

27. An electrical contact according to claim 19 wherein the alloy is neodymium in an amount from about 0.1 to 0.7 percent.

References Cited UNITED STATES PATENTS 2,539,298 1/1951 Doty et al. --173 R 2,669,512 2/1954 Larsen et al. 75-l73 R 2,673,167 3/1954 Vines 75-173 R 3,114,631 12/1963 Sistare et al. 75-173 R 3,472,654 10/1969 Comey et al. 75-l73 R 3,540,883 11/1970 Comey 75-173 R L. DEWAYNE RUTL-EDGE, Primary Examiner E. L. WEISE, Assistant Examiner 

